1. FIELD OF THE INVENTION
The magnetic sensor of the present invention relates to musical instruments, and, more particularly, to an electronic sensing device for use with musical instruments having magnetic vibrating elements.
2. DESCRIPTION OF THE PRIOR ART
For several years people have attempted to amplify the musical output of a piano, and the magnetic sensor of the present invention is believed to be more understandable with reference to prior piano amplification techniques.
With any attempt to obtain a louder sound out of a piano, there are basically two problems to be solved, viz., feedback and bleedthrough. Feedback occurs because the whole sounding board of the piano acts as a sound gatherer for the amplification device, while bleedthrough occurs when the outputs of other instruments used in proxmity to the piano are picked up by the piano amplifying device. Bleedthrough might occur, for example, when a piano is used in a band or orchestra, and an amplification device is used on the piano.
Early attempts at piano amplification utilized conventional vocal microphones to amplify the sound output of the piano. This type of piano amplification proved undesirable for two reasons. First, a piano is a distributed sound source instrument as opposed to a point source instrument, like a trumpet. The piano sound is transferred to the surrounding air by the very large area of its sounding board. This sounding board usually requires no one point to be very loud, and it was found to be exceedly difficult to position a vocal microphone to adequately amplify the output of a piano. Secondly, if a plurality of microphones were utilized to amplify only specific portions of the output of the piano, then the phenomenon of phase-cancellation could very likely occur. Phase cancellation occurs when one note arrives at two microphones at slightly different times by traveling different distances to get to each microphone. The output of one microphone would, in these instances, tend to cancel the output of another microphone. Consequently, this amplification technique for a piano has not proven desirable.
Another atttempt to amplify the sound output of a piano consisted of the utilization of "contact microphones." These contact microphones were designed to be inserted between the sounding board of the piano and a support post in back of the piano. A wire emerged from the contact microphone for insertion into a suitable amplification device. Most of the contact transducers originally manufactured employed a dynamic microphone construction, but this construction technique has been abandoned today in favor of piezoelectric crystal devices. These crystal devices are popularly referred to in the art as transducers, and utilize construction techniques similar to those used in spring gauges and seismic sensors. These transducers, as well as their dynamic microphone predecessors, have proven ineffective in the amplification of the piano output. The reasons for this ineffectiveness are that it is difficult to locate the contact microphone for effective piano reproduction and that these transducers tend to pick up the sound output of other musical instruments used in proximity to the piano (bleedthrough).
In the 1960's, the electric guitar began to be used by many musical entertainment groups, and the amplification technique utilized by electric guitars proved to be the inspiration for effective piano amplification. The amplification technique employed by electric guitars is to sense only the vibrations of the string, and many of the feedback and bleedthrough problems became minimal by using this technique. The concept of sensing only the vibration of the piano strings did not meet with universal acceptance, however, because many thought that the "true acoustical sound" of a piano could only be obtained from dispersion of the piano sound from its wooden sounding board.
This so-called "warmth of the wood" rationale has largely been discredited today for several reasons. First, for all practical purposes the fundamental and harmonic frequency of every musical note of the amplifier are contained in the vibrating string. Secondly, the sounding board system is merely a mechanical amplifier, and the difference in sound between a well-constructed piano and a cheaper model resides in how well the wooden sounding board transfers these string vibrations into the surrounding air. Thus, it is a generally accepted principal today that the wooden sounding board can add little or nothing to the sound output of a piano, but it can only substract from the original perfect balance in varying degrees. Therefore, whatever "warmth" is present in the wooden sounding board of the piano must by necessity also be present in the piano strings when actuated.
One of the prior attempts at piano amplification was an electrostatic string sensing device. This device was designed for grand pianos and consisted of utilizing an electrostatic pickup to detect the vibrations of the strings of the piano. A typical electrostatic pickup consisted of a four foot rod to suspend metal plates above the strings. These plates were charged to about 200 volts, and the strings of the piano were grounded. A capacitance existed, therefore, between the string and the plates, and any movement of the strings amounted to a change in this capacitance. This change in capacitance produced a variation signal on the plates. The outputs of the plates were coupled to a mixing device which extracted the variation signal from the 200 volt bias of the plate, and suitable amplification was then applied to the extracted variation signals.
The electrostatic sensing systems have several inherent disadvantages which have resulted in them not being widely utilized as a piano amplifier. Since the charged plates and strings formed a capacitance, the performance of the pickup was subject to weather conditions. The humidity, for example, could affect the value of capacitance between the plate and strings. Consequently, when the mixing device was designed to extract a 200 volt level from the outputs of the plate, then the value of the extracted variation signal could vary greatly. Furthermore, the metal plates were hard to shield and were subject to radio frequency interference. Also, it should be apparent from the above discussion that the electrostatic sensing devices required a high voltage power supply for proper operation.
The next approach to piano amplification was also designed primarily for use on grand pianos and consisted of sensing the vibration of the strings with a plurality of magnetic pickups. One such pickup which is still utilized is the Helpinstill piano pickup Model 175 which is produced by Helpinstill Designs of Houston, Texas. This piano pickup resembles the electrostatic sensing devices in that both devices sense all the strings with a framework that is placed across the same area of the piano. However, in the case of the magnetic string sensing device, coil-magnet assemblies are utilized in the pickup device instead of charged metal plates. Uniform pickups of any string underneath the whole length of the coil-magnet is realized without the necessity of a magnet and coil for each string.
As aforementioned, the magnet string sensing devices were originally designed for utilization on grand pianos, and they have enjoyed a not insubstantial amount of success for this purpose. It has been found that many entertainment groups are unable to use the magnetic pickup. The reason for this is simple, viz., they do not own or have a grand piano at their disposal. Efforts have been made to incorporate the magnetic sensing device of the Helpinstill Model 175 into other types of pianos besides grand pianos. This proved to be a laborious and often unsuccessful task and has not been actively pursued.
In the course of trying to modify other pianos to accept the Helpinstill Model 175 magnetic sensing system, it was discovered that the rubber strip magnets utilized in the coil-magnet assemblies of the Model 175 would magnetically adhere to the framework of a piano. This discovery led to the development of the magnetic sensor of the present invention and to the method of adapting an upright piano for electronic amplification of the present invention.